FIG. 43 shows a general servo motor driver used in a servo control system. The servo motor driver comprises a rectifying circuit 2 for rectifying a 3-phase alternating current given from a 3-phase alternating current source 1, a smoothing capacitor 3 for smoothing a rectified output from the rectifying circuit 2, a semiconductor switching circuit 4 receiving an DC current smoothed by the smoothing capacitor 3 as an input and constituting an output section of a PWM circuit outputting the 3-phase AC current having been subjected to PWM conversion to an AC servo motor 8, a current detecting section 5 for detecting from a chant resistor CT values of U-phase and V-phase currents each flowing into the AC servo motor 8 from the semiconductor switching circuit 4, an A/D converter 6 for converting an analog voltage signal proportional to a current value detected by the current detecting section 5 to a digital signal, and a CPU 7 fetching a digital signal outputted from the A/D converter 6 as a current feedback signal, executing a specified computing operation according to a current feedback value and a speed command value, and outputting a PWM signal for each phase to a base terminal for each phase in the semiconductor switching circuit 4.
It should be noted that, in this specification, a circuit comprising the rectifying circuit 2 and the smoothing capacitor 3 is called converter.
FIG. 44 shows a general servo control system. Servo control system comprises a positional control section 11 for generating a command concerning speed from a difference between a positional command value outputted from the NC unit 10 and a positional feedback value from a motor edge or a machine edge, a speed control section 12 for generating a command concerning a current from a difference between a speed command value from the positional control section 11 and a speed feedback value, and a current control section 13 for generating a current provided to the servo motor 8 from a difference between a current command value from the speed control section 12 and a current feedback value from a chant resistor CT. The servo motor 8 rotates and drives a feed screw 16 with a reduction gear 15, and linearly moves a table 17 which is an object to be controlled.
In a case of semi-closed loop, a motor edge detector 18 connected to a servo motor 14 and detecting a rotational speed and a rotational angle of the servo motor 8 and a position of magnetic pole is used, and in a case of full-closed loop, a machine edge position detector 19 for detecting a position for linear movement of the table 17 is used, and a feedback signal is obtained with the detectors.
FIG. 45 shows a current control section in an AC servo motor. This current control section comprises a speed control section 20 for outputting a q-axis current command value, a limiter 21 for limiting the q-axis current command value outputted from the speed control section 20 to protect a semiconductor element constituting a PWM modulating section 25 described later, a d-q coordinate converting section 22 for converting U-phase and V-phase currents outputted to the AC servo motor 8 to d-axis and q-axis currents, a current controller 23a for receiving a difference between a d-axis current command value and a d-axis current value outputted from the d-q coordinate converting section 12 given thereto and generating a command concerning d-axis voltage so that the difference is zeroed, a current controller 23b for receiving a difference between a q-axis current command value outputted from the speed control section 20 and a q-axis current value outputted from the d-q coordinate converting section 22 given thereto and generating an command for q-axis voltage so that the difference is zeroed, a 3-phase converting section 24 for subjecting d-axis and q-axis voltage commands outputted from the current controllers 23a and 23b to three-phase conversion to generate commands for voltages in U, V and W phases, and a PWM modulating section 25 for generating a 3-phase AC current to be provided to the AC servo motor 8 from the voltage commands for each phase outputted from the three-phase converting section 24.
In the configuration as described above, when the NC unit 10 outputs a positional command value, an command concerning speed is generated by the position control section 11 so that the positional command value coincides with a positional feedback value from a motor edge position detector 18 in case of a semi-closed loop or from a machine edge position detector 19 in case of a full-closed loop, and a q-axis current command is generated by the speed controller 12 so that the speed command value will coincide with the speed feedback value detected by the motor edge position detector 18. In contrast to the q-axis current command, zero (0) is always given to the d-axis current command.
To follow the command, the 3-phase AC current for driving the AC servo motor 8 is finally subjected to PWM modulation by and outputted from the semiconductor switching circuit 4 shown in FIG. 43 according to the q-axis current command and d-axis current command.
Next, a description is made for operations of a protecting function in the conventional type of servo motor driver as described above.
As described above, when a q-axis current command is generated to follow a positional command from the NC unit 10, if the command value exceeds an allowable current value for the semiconductor switching circuit 4, controls are provided by the limiter 21 to limit the current.
Also if a percentage of a current command value or a phase current value detected by the current detecting section 5 against an allowable current value exceeds a certain level and the state continues for a prespecified period of time, a specific alarm is generated to indicate that the difference is excessive.
Also if a difference between a positional command value given to the positional control section and a positional feedback value is larger than a value deviated by a certain percentage against a logically computed deviation, an alarm indicating that the deviation is excessive is issued.
Also a current flowing in each transistor in the semiconductor switching circuit 4 is detected, and if it is determined that the current is excessive or a gate shutdown request from a multi-shaft driver or a converter section, or from an NC unit, a gate for each transistor in the semiconductor switching circuit is shut down by hardware.
In a full-closed loop having the machine edge detector 19, if a difference between a resolution as well as a number of pulses from the machine edge position detector 19 and a number of pulses presumably computed according to a resolution of and a number of pulses from the motor edge position detector 18 so that in normal operating mode a machine edge and a motor edge coincide with each other and the difference will be zero exceeds an allowable value, it is determined that there is abnormality in feedback, and a prespecified alarm is generated.
In the conventional type of servo motor driver, there is no function to monitor a pattern of a current wave form of a current in each phase flowing in the motor output terminal, so that misconnection of the motor output terminal can not be detected, and there is no way but to provide an alarm indicating excessive difference or excessive load for stopping system operation, which is disadvantageous.
Also the conventional type of servo motor driver does not have a function to detect a voltage in each phase loaded to the motor output terminal, so that it is disadvantageously impossible to detect that connection of the motor output terminal or that of a bus-bar for a converter has not been established.
Also the conventional type of servo motor driver does not have a function to monitor a gate shutdown signal to a transistor in an inverter section for controlling a current for a servo motor, nor a function to monitor a type of gate shutdown request signal, so that it is disadvantageously impossible to more detailedly determining causes for an alarm such as that for an excessive difference or feedback abnormality.
Also the conventional type of servo motor driver does not have a function to more detailedly determine, when an alarm indicating an excessive difference or an excessive load, or abnormality in feedback is generated in a full-closed loop having a machine edge position detector, causes for the alarm, and there is no way but to stop system operation, or a long time is required to find out the cause, which is disadvantageous.
Also the conventional type of servo motor river does not have a means for making a determination, when an alarm indicating an excessive difference or an excessive load is generated during acceleration or deceleration, as to whether the alarm has been generated because of a time constant for acceleration or deceleration is too small, or because an abnormal load has been generated, and for this reason there is no way but to stop system operation even in a case where the system operation could be continued by automatically readjusting related parameters.
Also the conventional type of servo motor driver does not have a function to recognize a type of detector, so that if actual specifications for communications of a detector set with parameters are different from those of a detector actually connected thereto, a no-signal alarm is generated and the state may be mistaken as a fault in the detector or the connection cable, and in addition a long time is required for restoration, and sometimes even very simple mistakes in parameter setting may disable system operation.